Electronic circuits each having a switching regulator for feeding back an output voltage to turn on or off a MOS transistor, thereby controlling the output voltage such that the output value becomes a target voltage have been generally used.
For example, a switching regulator has a feedback loop, and includes a differential amplifier provided as an error amplifier on the path of the feedback loop. An output voltage is input as a feedback voltage to the differential amplifier. The differential amplifier outputs a differential voltage which is obtained by amplifying a difference between the feedback voltage and a reference voltage. The differential voltage is input to a comparator.
Also, the switching regulator has a current sense amplifier for detecting an input current. The input current is converted into a voltage, which is input as an input conversion voltage to the comparator. As a result, the comparator compares the input conversion voltage and the differential voltage, and turns on or off the MOS transistor according to the comparison result. This feedback control causes the output voltage to be maintained constant. However, actually, since the output voltage pulsates according to ON/OFF control on the transistor, in order to smooth the output voltage, a capacitor and an inductor are provided at the output of the switching regulator.
Further, the switching regulator has a phase compensation circuit. The phase compensation circuit is composed mainly of a capacitor and resistors, and lowers the gain of an output signal in terms of the alternating current relative to an input signal. Also, the phase compensation circuit adjusts the phase delay of the output signal relative to the input signal. That is, the phase compensation circuit adjusts the relation between the gain and phase of the output signal relative to the input signal. Specifically, due to influence of the capacitor for smoothing and the capacitor for phase compensation, a first pole and a second pole occur in a Bode plot, and at each pole, the phase is delayed by 90 degrees (total 180 degrees). Further, due to the resistors, the phase advances by 90 degrees. Therefore, before the phase is delayed by 180 degrees, the phase compensation circuit adjusts the gain such that the gain becomes 0 dB or lower. As a result, it is possible to prevent oscillation of the output voltage.
Here, in a case of providing a phase compensation circuit inside a switching regulator circuit, selection of internal elements, such as a capacitor, of the phase compensation circuit may need a lot of time. Also, in a case where the capacity of the capacitor of the phase compensation circuit is relatively large, ON/OFF control on a MOS transistor which is a response to a change in an output voltage is delayed. As an example for solving this problem, a DC to DC converter control circuit has been disclosed in Japanese Patent Application Laid-Open No. 2007-174772. The DC to DC converter control circuit controls an output voltage such that the output voltage becomes a target voltage, without providing a differential amplifier and a phase compensation circuit in the circuit.
However, the DC to DC converter control circuit disclosed in Japanese Patent Application Laid-Open No. 2007-174772 is a circuit which is controlled in a “voltage mode”. Here, the voltage mode means a mode for controlling the output voltage of the control circuit on the basis of change of an input voltage such that the output voltage becomes close to target voltage. The DC to DC converter control circuit which is disclosed in Japanese Patent Application Laid-Open No. 2007-174772 and is controlled in the voltage mode as described above can eliminate one pole by removing the differential amplifier and the phase compensation circuit. However, due to a double pole of a capacitor for smoothing for the voltage mode, and a coil which is connected to the output of the transistor, the phase is delayed by 180 degrees, and thus the output voltage becomes likely to oscillate. As a measure for stabilizing the output, a measure such as provision of another phase compensation circuit becomes necessary. As a result, design becomes complicated, and due to provision of new components into the circuit, the size of the circuit increases.
Also, in a case where an element, such as a differential amplifier, for increasing the gain of the entire circuit (hereinafter, referred to as a gain increasing element) is not provided, there are the following problems. That is, if it is assumed that the DC gain of the entire circuit in a case where there is a gain increasing element provided in the circuit is, for example, 1000 times, due to an increase in an output current Iout, the DC gain of the entire circuit decreases. For example, due to an increase in the output current Iout, the gain decreases to a half, that is, the gain changes from 1000 times to 500 times. As a result, a voltage difference between the non-inverted input terminal and inverted input terminal of a comparator of the inside of the circuit specifically becomes as follows according to a voltage difference equation ([VOLTAGE DIFFERENCE]=Vout/A, wherein A represents a gain). In a case where a voltage which is input to the non-inverted input terminal of the comparator is 1 V, and a voltage which is input to the inverted input terminal is 1 V, the voltage difference between both terminals becomes 0 V, and a difference does not occur. However, actually, due to a difference in characteristics between both terminals, even in a case where there is no difference between input voltages, a voltage difference occurs between both terminals. For example, according to the voltage difference equation, in a case where the gain A is 1000, the voltage difference becomes 1 mV, and in a case where the gain A is 500, the voltage difference becomes 2 my. Here, in a case where the output current Iout increases, the DC gain of the entire circuit decreases due to the following reason. In a case where the output voltage Vout is substantially constant, as the output current Iout increases, the resistance value of a load resistor Rd decreases on the basis of Ohm's law. Further, if the resistance value of the load resistor Rd decreases, the DC gain also decreases according to a gain relation equation (A=Gm×R, wherein Gm represents conductance, and R represents resistance).
Also, in a case where both input terminals of the comparator has a voltage difference as described above, when the voltage which is input to the non-inverted input terminal of the comparator, and the reference value which is input to the inverted input terminal are compared, even if a difference between both voltage values becomes 0 V, the voltage difference between both input terminals of the comparator according to the decrease of the gain appears as the voltage difference of a target voltage Vtar from the output voltage Vout. However, in a case where the DC gain of the entire circuit is high to a certain degree as described above, even if the DC gain decreases according to an increase in the output current Iout, the voltage difference between both input terminals of the comparator becomes a relatively small value, and the voltage difference of the output voltage Vout from the target voltage Vtar also becomes a small value.
Further, since the DC gain of the entire circuit decreases due to an increase in the output voltage Vout, the voltage difference between both input terminals of the comparator becomes higher than that before the decreasing of the DC gain, and this voltage difference occurs as the voltage difference between the output voltage Vout and the target voltage Vtar.
Here, if the gain increasing element is removed from the inside of the circuit, the gain of the entire circuit further decreases. If the gain increasing element is removed from the inside of the circuit, for example, the DC gain of the entire circuit decreases from 1000 times which is the DC gain before the removal, to 100 times. In this case, if the output current Iout increases, whereby the DC gain further decreases (for example, from 100 times to 50 times), when the DC gain A is 100, the voltage difference becomes 10 mV, and when the DC gain A is 50, the voltage difference becomes 20 mV.
Also, of a voltage difference (2−1=1 mV) between both input terminals of the comparator due to an increase of the output current Iout in a case where there is a gain increasing element in the circuit, and a voltage difference (20−10=10 mV) in a case where there is no gain increasing element in the circuit), the voltage difference in the case where there is no gain increasing element in the circuit becomes larger. Therefore, the voltage difference of the output voltage Vout from the target voltage Vtar in the case where there is no gain increasing element in the circuit becomes larger than that in the case where there is a gain increasing element in the circuit, and thus stable power supply to a load may be impossible.